The present invention relates to a nickel cermet and the relative process for its preparation.
The nickel cermet can be used as an anodic material for solid oxide fuel cells (SOFC).
Solid oxide fuel cells (SOFC) using as solid electrolyte a mixture of yttrium oxide (Y2O3) and zirconium oxide (zirconia) (ZrO2), an anode consisting of a nickel/zirconium oxide and a cathode of lanthanum manganite (LaMnO3), are known in the art. (See Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A12, Ed.1989, pages 80-82).
The nickel/zirconium oxide cermet used as anodic material normally consists of a dispersion of metal nickel in zirconium oxide stabilized in cubic crystalline form with yttrium oxide.
Processes for the preparation of this cermet essentially comprise the mechanical dispersion of nickel oxide in stabilized zirconium oxide, followed by the reduction of the nickel oxide to metal nickel. For example, according to the description of U.S. Pat. No. 3,300,344, mixed zirconium and yttrium oxides, obtained by precipitation from a water solution, are shaped with the addition of nickel oxide and carbon in powder form and the nickel oxide is then reduced to metal nickel by effect of the carbon at a high temperature. In addition the article xe2x80x9cMorphology and Electrochemistry of Porous Nickel/Zirconium Cermetsxe2x80x9d, (Proceedings of the First Internation Symposium on Solid Oxide Fuel Cells, S. C. Singhal, Ed. 1989, pages 90-98) describes reduction with hydrogen of a mixture of compact powders obtained by co-grinding, with a sphere mill, nickel oxide and stabilized zirconium oxide. Other known techniques are those based on C.V.D. (Chemical Vapour Deposition) and P.S. (Plasma Spraying), described for example by H. Arai in International Symposium on SOFC, Nov. 13-14, 1989, Nagoya, Japan.
The nickel cermets obtained with the processes of the known art are not entirely satisfactory in applications in fuel cells, mainly owing to their incapacity to give the nickel a sufficiently high active area as to offer interesting catalytic properties (A. L. Lee et al., Ind. Eng.Chem.Res., 1990, 29, 766-773).
Another problem derives from the difficulty in obtaining the complete reduction of the nickel oxide particles to metal nickel. S. D. Singhal""s article, mentioned above, specifies in fact that particles with a size of more than 3 xcexcm conserve a NiO nucleus after reduction with hydrogen. As far as the C.V.D. and P.S. techniques are concerned, there are problems relating to both the quality of the material obtained and difficulties in the practical application of the methods themselves in the construction of medium and high power cells.
A process has recently been described in the U.S. Pat. No. 5,261,944, which enables the production of a solid of nickel oxide and stabilized zirconium oxide, in the form of two distinct phases, with a phase distribution at a level of less than 1 xcexcm. In addition, the nickel oxide contained in this solid can be completely, or almost completely, reduced by means of hydrogen to give a nickel cermet with a submicronic phase distribution and with a high active area of the nickel, thus making the cermet particularly suitable as a material for solid oxide fuel cells (SOFC).
We have now found a process for the preparation of a nickel cermet which enables a product to be obtained with an extremely high conductivity than that of the cermet described in the above patent.
The nickel cermet of the present invention consists of from 35 to 70% by weight of a metal nickel phase and 65 to 30% by weight of a zirconium oxide (zirconia) phase stabilized in cubic form by 5 to 20 moles of yttrium oxide (yttria) for each 100 moles of zirconium oxide, the two phases, upon X ray diffractometric analysis, being distinct and homogeneously distributed, and is obtained by a preparation process comprising the following steps:
a) preparation of a suspension of zirconia stabilized by yttria (YSZ), by dispersing zirconia stabilized by yttria (YSZ), having a particle size of between 1 and 40 mxcexc, preferably between 15 and 20 mxcexc, in dimineralized water;
b) preparation of a solution of nickel by dissolving a hydrosoluble and thermodecomposable nickel salt in demineralized water;
c) mixing of the nickel solution with the dispersion obtained in step (a) and homogenization of the resulting dispersion by magnetic stirring for a time of between 5 and 40 minutes, preferably between 10 and 30 minutes;
d) elimination of the water from the dispersion obtained in step (c) by a system capable of operating without, or basically without, decomposition phenomena obtaining a solid consisting of particles of zirconia stabilized by yttra (YSZ) covered with the nickel salt;
e) calcination under oxidating conditions of the solid obtained in step (d) to cause the formation of a solid consisting of particles of YSZ covered with nickel oxide;
f) treatment with hydrogen of the solid obtained in step (e) to reduce the nickel oxide to metal nickel.
This nickel cermet normally has a surface percentage covered by nickel of between 4 and 30 or more.
The hydrosoluble and thermodecomposable nickel salts used to form the nickel solution can be selected from all the salts of carboxylic acids and nitrate salts: Ni(CH3COO)26H2O and Ni(NO)36H2O are particularly suitable.
The solution of the nickel salt and the dispersion of zirconia stabilized by yttria are prepared and then mixed so as to have in the nickel cermet finally obtained, from 35 to 70% by weight of metal nickel and from 65 to 30% by weight of zirconium oxide stabilized by from 5 to 20 moles of yttrium oxide for every 100 moles of zirconium oxide.
According to the present invention, the water is eliminated (step d) from the dispersion obtained in step (c), as specified above, under such temperature conditions as to basically avoid decomposition. In particular it is possible to operate by evaporating the water under vacuum at a temperature of not more than 80xc2x0 C., operating for example with a rotating evaporator. According to another embodiment, the water is eliminated by the spray-drying technique feeding the dispersion with a composition having a weight of about 5 to about 30% (w/w) into a spray-drying apparatus in equicurrent or countercurrent with an inert gas, such as N2, or air. The gas flow entering the spray-dryer will generally have a temperature of about 150-300xc2x0 C., preferably 170-200xc2x0 C., and the out-going current a temperature of about 90-200xc2x0 C., preferably 130-170xc2x0 C. According to another method, the water is eliminated by the use of a vacuum freeze dryer.
In any case a solid, porous and friable, generally ochre-green coloured solid is separated from the water removal step.
According to the present invention, the solid thus obtained is subjected to calcination, in step (e) of the process, operating at high temperatures and in an oxidating environment. In particular suitable calcination temperatures are generally between 800 and 1000xc2x0 C. and the calcination times can vary from 1 to 10 hours. According to a preferred embodiment the temperatures are about 900xc2x0 C. and the time 3-5 hours.
The oxidating environment can consist of oxygen, air or air enriched with oxygen. Operating under these conditions the combustible or decomposable fraction is eliminated from the solid and there is the formation of metal oxides present (Ni).
This solid is subjected to treatment with hydrogen in step (f) of the process, to reduce NiO to metal Ni and therefore obtain the cermet. In particular the reduction is carried out by putting the solid in contact with gaseous hydrogen operating at temperatures within the range of 20 and 1000xc2x0 C. to obtain the complete, or almost complete, reduction of the NiO to metal Ni. Useful reduction times are about 1-3 hours.
In this way the Ni cermet of the present invention is obtained, which generally contains 35-70% by weight of a metal nickel phase and 65-30% by weight of a Zr oxide (zirconia) phase stabilized in cubic form with yttrium oxide (yttria).
A further aspect of the present invention relates to the use of the nickel cermet described above as anodic material for solid oxide fuel cells (SOFC).
In particular this anode can be obtained by applying, with the known techniques, the nickel cermet of the present invention to a solid electrolyte of zirconium oxide stabilized with yttrium oxide. According to a particular embodiment the NiO/YSZ obtained after calcination in step (e) of the process, is applied to the solid electrolyte, followed by the reduction xe2x80x9cin situxe2x80x9d of the nickel oxide to metal nickel.
The following example illustrates the invention but does not limit its scope in any way.